Short note: Aperture antenna

An aperture antenna is one that radiates through an opening (aperture) whose size is several wavelengths, rather than from currents on thin wires. The fields across the aperture act as secondary sources (Huygens' principle), and the radiation pattern is found from the aperture field distribution.

 waveguide   aperture (E-field across opening)
  ______   .‾‾‾‾‾‾‾‾‾|
 |______|=<  horn    |  ===> beam
           '_________|

Examples

• Open-ended waveguide and horn antennas (sectoral, pyramidal, conical).
• Slot antennas cut in a metal sheet or waveguide wall.
• Reflector antennas (parabolic dish) – the dish mouth is the aperture.
• Lens antennas and microstrip patches (radiating edges).

Key relations

Effective aperture  A_e = η A    (η ≈ 0.5-0.8)
Gain                G = 4π A_e / λ²
Beamwidth           ≈ k λ / D   (k ≈ 50-70 degrees)

Features: high gain and narrow beam because the aperture is large compared with λ; wide bandwidth; easily flush-mounted on aircraft and vehicles; used mainly at microwave frequencies (above ~1 GHz). Applications: radar, satellite links, microwave relays, feeds for reflectors and gain standards.